1. Field of the Invention
This invention relates to improvements in waveform processors, and more particularly to improvements in digital signal processing of sigma delta modulated (SUM) pulse streams, and in one aspect to methods and circuits for mixed analog/digital processing of one or more delta modulated (DM) pulse streams.
2. Relevent Background
Traditionally, DM has been used in military applications, particularly in adaptive delta modulation (ADM). Recent advances in semiconductor technology and in charged coupled capacitor devices led many integrated circuit (IC) producers, such as Motorola, Analog Devices Corporation, Crystal, and others, to successfully design chips for a variety of delta modulators. The basic idea is to use a highly oversampled linear delta modulator (LDM) to achieve a high signal-to-noise ratio. A high frequency (in the order of MHz) pulse density stream is not suitable for direct conventional digital signal processing (DSP), and therefore, decimation is generally used to assure proper speed, in order to interface with existing DSP hardware.
In the past, there have been attempts to process the DM pulse stream in a direct fashion. To achieve such direct processing, specialized hardware is needed. One known approach for addition of two DM sequences using a delta adder (DA) was proposed by N. Kouvaras, "Operations on Delta Modulated Signals and their Applications in Realization of Digital Filters," The Radio and Electronics Engineering, Vol. 48, No. 9, September, 1978, pp. 431-438, incorporated herein by reference. Therefore, the operation of a delta adder circuit is to add digital input signals X.sub.n and Y.sub.n applied to the input terminals. Assuming X.sub.n and Y.sub.n to be DM sequences, then, as shown in FIG. 1, the output sequence produced by the delta adder is also a binary DM sequence. According to Kouvaras' findings, a demodulated signal of sum S.sub.n results in an analog signal s(t) whose amplitude is equal to 1/2 (x(t)+y(t)), as shown in FIG. 1a.
Yet another attempt to process DM signals in a direct form was proposed by Massey, U.S. Pat. No. 4,450,532. Massey proposed a circuit arrangement for receiving an applied analog type electrical signal and generating a pulse train output signal whose average frequency is proportional to the square of the applied analog signal.
Lu, U.S. Pat. No. 4,622,649, proposed an improved convolution processor that requires no multiplication operations. Yamazaki, US patent 5,208,594, proposed using a multibit signal processing technique to process one bit DM pulse stream.
Simone, U.S. Pat. 3,314,015, synthesized an "artificial transfer network", having a desired transfer response, employing high speed digital and analog techniques.
Nishino, et al., U.S. Pat. No. 4,730,165, proposed a nonlinear signal processing apparatus employing analog signal processing techniques. The time varying input signal is nonlinearly processed by a differential circuit and then arithmetically combined with the input signal.
Yet another approach, described by Kikkert in U.S. Pat. No. 4,320,361, deals with mixed analog/digital signal of a DM pulse stream. The digital data is arranged to operate a switch which, in the case of amplitude modulation, produces carrier frequency pulses.